Electro dip lacquering is a known process which has often been described in the prior art, see for example European patent Nos. 4,090 and 66,859. It produces a uniform coating on various metal surfaces and thus protect them against corrosion. Subsequent layers can be applied to the first or primer layer thus applied. The general procedure involves dipping the electrically conductive parts into an aqueous electro dip bath, connecting them as cathode or anode and causing the lacquer to be coagulated on the surface of the substrate by the direct current. One advantage of the process is that when hollow bodies are being coated, the electric resistance increases on their external surface and surfaces to which there is no easy access, for example the inner parts or cavities which have only small openings, are to an increasing extent being coated by this process. The material which adheres to the surface is then heated so that it is physically caused to flow and it is optionally also cross-linked chemically so that a homogeneous, smooth, wear resistant surface is obtained.
One advantage of the process of electro dip lacquering is that it can also coat parts of surfaces which are difficult to reach. Thus, good protection against corrosion can also be obtained on these parts. The coating of cavities and edges can be facilitated by varying some of the deposition parameters. Major mechanical shaping processes are generally not carried out on their metal substrates, because the coating can crack and burst, whereby the protection against corrosion will be considerably reduced. It is precisely in those parts which are subject to mechanical stress that folds, cavities, and other coating discontinuities frequently occur and these are particularly likely to corrode.
It has been found necessary for certain applications to carry out mechanical shaping after coating and baking of the electro dip lacquer. This normally results in cracks and mechanical damage in the dense, baked electro dip lacquered surface. These defects then constitute vulnerable points where corrosion is like to occur. It would be possible to prevent this by applying a subsequent coating at these points but this procedure is complicated and cannot be carried out at every point of vulnerability.
Another known procedure involves coating the metallic substrate with an anodic electro dip lacquer coating compound. After cross-linking has taken place, these coatings can still withstand mechanical stresses to such an extent that the mechanical deformation will cause no damage to the surface of the film. These anodic electro dip lacquer coating compounds, however, have the disadvantage that they are inferior to cathodic electro dip lacquer coatings in the protection that they afford against corrosion. Moreover, the throwing power of the coating compounds, ie.e the possibility of coating cavities which are difficult to reach, is considerably inferior to that obtained in cathodic electro dip lacquering. Cathodic electro dip lacquering has therefore become the method of predominant choice. It is in the shaping of cathodically electro dip lacquer substrates, however, that the disadvantages described above occur.